FIG. 1 illustrates an exemplary wireless communication system. Referring to FIG. 1, a wireless communication system (100) includes a plurality of base stations (110) and a plurality of user equipments (or user terminals) (120). The wireless communication system (100) may include a homogeneous network or a heterogeneous network. Herein, the heterogeneous network refers to a network having different network entities, such as a macro cell, a femto cell, a pico cell, a relay station, and so on, co-exist therein. The base station generally corresponds to a fixed station communicating with a user equipment, and each base station (110a, 110b, and 110c) provides services to specific geographical areas (102a, 102b, and 102c). In order to enhance the system performance, the specific area may be divided into a plurality of smaller areas (104a, 104b, and 104c). Each of the smaller areas may be referred to as a cell, a sector, or a segment. In case of an IEEE 802.16 system, a Cell Identity (Cell_ID or IDCell) is assigned based upon the entire system. Conversely, a sector or segment identifier is assigned based upon the specific area, to which each base station provides service, and the sector or segment identifier is assigned with values ranging from 0 to 2. The user equipment (120) is generally distributed throughout the system and may be fixed or mobile. Each user equipment may communicate with one or more base stations via Uplink (UL) or Downlink (DL) at an arbitrary moment. The base station and the user equipment may perform communication with an FDMA (Frequency Division Multiple Access), a TDMA (Time Division Multiple Access), a CDMA (Code Division Multiple Access), an SC-FDMA (Single Carrier-FDMA), an MC-FDMA (Multi Carrier-FDMA), an OFDMA (Orthogonal Frequency Division Multiple Access), or a combination of the above. In this specification, an uplink refers to a communication link from a user equipment to a base station, and a downlink refers to a communication link from a base station to a user equipment.